A local craft store had that thin crafting rubbery sheets that peole use for rivers, but they had some texured brown ones, which I think would make good fields, though I am not sure whether they have any longevity (from heat and light).

These fields look really great. I've been checking out various carpet tiles for use as cultivated fields, which I can then cut to hex shape and stick on hex boards. However, your fields are thinnner in depth and far more effective than the results I've achieved so far!